Generally, a material which is capable of shielding magnetic fields such as an external magnetic field, the earth magnetic field or the like is called magnetic shield. One example of the magnetic shields is the inner shield installed within a cathode ray tube 10 as shown in FIG. 1. If external magnetic fields such as the earth magnetic field or the like intrude into the cathode ray tube 10, then the electron beams are deflected from their paths, with the result that they cannot land on the relevant pixels after passing through a shadow mask 15. Consequently, a color spreading occurs on the screen so as to aggravate the picture quality. Therefore, it is necessary that the external magnetic fields be shielded to prevent straying of the electron beams. The inner shield is used for this very purpose.
In FIG. 1, reference code 14 indicates a frame, and 16 indicates a phosphor screen.
The cold rolled steel sheet for the inner shield is roughly classified into a soft material and a hard material. That is, in the inner shield manufacturing methods, there is the case where only a bending is carried out, and the case where a deep drawing is carried out. In the case where the bending is carried out, the deformation amount is not very large, and therefore, the formability is not very much required, so that a cold rolled steel sheet can be used without carrying out a recrystallization annealing. This material is called "hard material". In the case of the hard material, a cold rolled steel sheet is bent, and the recrystallization is made to occur during the blackening process, thereby securing the magnetic properties. In contrast to this, in the case where the deep drawing is carried out, a considerable amount of deformation is imposed, and therefore, the formability has to be superior. Because of this fact, a recrystallization annealing is carried out so as to improve the formability. However, in the case of the soft material, the formability is superior owing to the execution of the recrystallization annealing, but there is the disadvantage that the addition of the process step is accompanied by an increase of the manufacturing cost.
The most important characteristic which is required for the cold rolled inner shield steel sheet of the soft and hard kinds is the magnetic field shielding property. This property is decided by the permeability (.mu.) and the coercive force (Hc). If the magnetic field shielding capability is to be ensured, there are required a high purity steel having a low impurity content, and a highly clean steel having a low level of non-metallic inclusions. Further, the grain size has to be made coarse during the manufacturing process. The conventional techniques which satisfy the above described requisites include (1) a decarburization annealing method, (2) a low temperature hot rolling method and (3) a strain annealing method.
(1) The decarburization annealing method is described in Japanese Patent Sho-62-280329. In this technique, a steel with a carbon content of 0.02% or less is used to carry out a hot rolling by adopting a low slab reheating temperature. Then a first cold rolling is carried out at a reduction ratio of 60% or more, and then, a decarburization annealing is carried out to lower the carbon content to 0.003% or less. Then a second cold rolling is carried out at a reduction ratio of 60% or less, and then, a final annealing is carried out at a temperature of above 650.degree. C. In this method, after the first cold rolling, the decarburization is carried out to lower the carbon content so as to satisfy the required characteristics. In this method, however, a special decarburization facility such as the OCA (open coil annealing) is required, this being a disadvantage.
(2) In order to solve the above described problem of the decarburization method, Japanese Patent Hei-166230 discloses a low temperature hot rolling method. In this technique, 0.005-0.08 wt % of Ti is added into a steel with a carbon content of 0.005 wt %, and then, a low temperature hot rolling is carried out. Then a cold rolling is carried out, and then, an annealing is carried out at a temperature of above 620.degree. C. In this technique, there is the advantage that a single step of cold rolling is carried out. However, the cold rolling reduction ratio is very high, and therefore, the grain size in the final product is very fine, and therefore, the magnetic properties are not superior. That is, as described in the example, the coercive force is more than 1.75 Oe, and therefore, a steel sheet with superior magnetic properties cannot be obtained.
Further, the expensive element Ti is added, and the hot rolling temperature is lower than the usual hot rolling temperature (720-800.degree. C.). Accordingly, it is not easy to apply the method to a continuous hot rolling process.
(3) In order to solve the above described problems, Korean Patent Application No. 97-714422 proposes a strain annealing method. In this technique, a cold rolling and a recrystallization annealing are carried out, and then, a skin pass rolling is carried out. Then a strain annealing is carried out at a temperature of 660 720.degree. C. In the case of this method, a coarse grain size can be obtained in spite of a single step of the cold rolling, and therefore, a cold rolled steel sheet with superior magnetic properties can be manufactured. However, when carrying out the strain annealing, the annealing temperature is relatively high, and therefore, the sticking phenomenon may occur during the batch annealing.
In order to solve the problems of the single round cold rolling method, Japanese Patent Gazette No. Sho-60-255924 proposes a method in which the cold rolling is carried out two times or more. In this method, a steel with a carbon content of 0.08% is used to carry out a hot rolling and a first cold rolling. Then a decarburization annealing is carried out to produce a recrystallized product with a carbon content of 0.01% or less. Thereafter, a second cold rolling is carried out at a reduction ratio of 5-17%, and then, a second annealing is carried out at a temperature of 680-800.degree. C. Then finally a third cold rolling is carried out at a reduction ratio of 50% or more. In this technique, the magnetic properties are superior, but three steps of cold rolling and two steps of decarburization annealing and recrystallization annealing have to be carried out, with the result that this complicated method causes an increase in the manufacturing cost.